Photoshop CS6 & Screen Resolution: How Can I See Accurate Measurements?

How can I tell adobe exactly what "1 inch" looks like? I would have to reduce my screen resolution to something very low so that when I put a ruler up to my screen, it would match what Adobe says is "1 inch".

Not sure what exactly you're trying to do. Placing a ruler against the screen really won't tell you anything as you'll get different results based on your screen resolution, document resolution, level of magnification, etc.

Photoshop gives you lots of tools to determine how big things will be when they print. Rulers are the simplest of the bunch (Ctl/Command-R). Think of it as placing a digital ruler against your screen. You can see exactly how large your printed elements will be, with Photoshop compensating for resolution and magnification.

Well, there's View > Print size, where Photoshop will do its best to show you the size an image will be when you print it to paper. But for Photoshop to automatically display things at their real-world size? To the best of my knowledge, there's no automated way to do this. To use your example, if you open a photo of a quarter, Photoshop doesn't know A.) it's a quarter and B.) how big it's supposed to be. All it sees is a grid of pixels.

I suppose if you know your monitor's native resolution (usually 72ppi or 96ppi) and set up a new document at that resolution, you could get 1 monitor inch to roughly equal 1 real-world inch, but you'd be locked into only viewing at 100% magnification, and anything you print will be jagged and pixelated. Plus you'd still have to do scaling of any element you brought into the document to get it to the right size.

By and large, Photoshop doesn't care about inches, or any physical measurement. All it cares about are the number of pixels.

Is that the diagonal measurement of the display? If so you'll need to convert it to either the vertical or horizontal dimension of the display. I'd recommend horizontal which has a 1:1.14734748441786 ratio to the diagonal (god bless trigonometry). Meaning the horizontal measurement of your display should be about 15.08" (rounded). Dividing 1920 pixels into that gives us 127.33567457123 or roughly 127ppi or 128ppi.

This value can be entered in Photoshop's Preferences under Units & Rulers in the field titled Screen Resolution.

All of your Rulers should be accurate once you've set this value properly and gone to View > Print Size for the document you want to see at size.

I'd recommend checking Photoshop's rulers against a real ruler and making small adjustments to the ppi as you deem necessary.